| The aim of this study was to characterize the physical deterioration and behaviour of a fibre-reinforced composite under cyclic loading. To investigate the effects of matrix dominated shear fracture, cyclic tests with a stress ratio R = 0.1 were performed on unidirectional 10° off axis graphite-bismaleimide (BMI) coupons. Static tests, including preliminary creep tests, were also performed to compare the damage evolution under different loading conditions. In all cases, strain gauges were mounted on the specimen surface in order to monitor the stress-strain response and modulus changes in selected material directions. Micro-cracking was monitored using acetate replicas of the specimen edge and surface. Microscopic evaluation was also performed on the fracture surface to give additional information.; Longitudinal debond cracks at the fibre-matrix interface were found to be the dominant fracture mechanism. The presence of hackles on the fracture surface indicated shear dominant cracking. The shear moduli and strains were found to vary significantly during testing. Using this data, different damage parameters in the material coordinate directions were assessed. The damage parameters based on shear moduli were found to be the most indicative of damage evolution. Damage increased quickly and tended to saturate under cyclic loading conditions after approximately 20% life, indicating that a dominant fatigue crack had initiated and damage was no longer uniformly distributed. Cyclic specimens with longer lives showed larger amounts of uniform longitudinal cracking and shear modulus degradation. Static results indicated that with increasing stress, the damage based on longitudinal crack growth and on shear modulus increased. One unique feature found through microscopic examination of the static and cyclic fracture surfaces was that cyclic specimens had bare fibres exposed due to fretting of the crack faces. |
